1. Field of the Invention
The present invention relates to an MR sensor and a flux guide which are stabilized by hard bias layers to prevent Barkhausen noise and which are joined by a contiguous self-aligned junction so as to provide a predictable overlap of the flux guide on a back end portion of the MR sensor thereby optimizing signal flux density in the MR sensor.
2. Description of the Related Art
A magnetoresistive (MR) sensor is employed in an MR read head for sensing magnetic fields on a magnetic storage medium, such as a rotating magnetic disk. The MR read head is carried on a slider mounted on a suspension. The suspension, in turn, is mounted to an actuator. The suspension biases the slider toward a surface of the disk. When the disk rotates, the loading is counterbalanced by a cushion of air (an "air bearing") generated by the rotating disk. The actuator moves the head to selected information tracks on the rotating magnetic disk. The resistance of the MR sensor changes in proportion to the change in magnetic field intensity caused by rotation of the disk. When a sense current is conducted through the MR sensor, the current changes in proportion to the change in resistance. Changes in the sense current are processed by a processor to produce playback signals corresponding to the information stored on the magnetic disk.
An MR sensor includes a stripe of MR material sandwiched between a pair of very thin insulative gap layers which are, in turn, sandwiched between a pair of magnetically conductive shield layers. Magnetic flux reaching the MR sensor extends through the gap layers to the shield layers. The MR sensor has an exposed edge at an air bearing surface (ABS) of the slider on which it is carried. The exposed edge interfaces with the air bearing. The MR sensor also has a back edge which is normally parallel to the air bearing surface and is embedded within the MR head. The magnitude of magnetic flux that reaches the MR sensor is at a maximum at the ABS. This magnitude decays along the MR stripe height and into the shields with a characteristic decay length. A boundary condition requires the flux magnitude to be zero at the back edge. When the stripe height of the MR sensor from the ABS to the back edge is less than the decay length, which will be described hereinafter, the flux loss along the height of the MR sensor is linear.
In the invention, which will be described in detail hereinafter, a flux guide is connected at the back edge of the MR sensor and extends away from the ABS so that only a portion of the magnetic flux reaching the MR sensor extends through the gap layers into the shield layers. With this arrangement a greater amount of flux is sensed in the MR sensor, thereby increasing the magnitude of the readback signal. Optimization of this signal is dependent upon the quality of the junction between the back edge of the MR sensor and the flux guide. The flux guide is a bilayer component comprising an insulation material layer and a flux guide material layer. The junction requires that the insulation material layer be located between the MR sensor and the flux guide material layer. Since the flux is reluctant to flow into the flux guide through an abutting junction at the back edge of the MR sensor it is necessary that the flux guide overlap an end portion of the MR sensor adjacent the back edge. It is also necessary that this overlap be precise. The amount of overlap is dependent upon such factors as the stripe height of the MR sensor and the risk of shorting of the sense current due to possible pinholes in the gap layers. The overlap is typically 0.1 .mu.m for a 1.0 .mu.m high MR sensor. When there is no overlap there is insufficient flow of flux into the flux guide and when the overlap is too long flux is conducted out of the MR sensor prematurely and the risk of shorting due to pinholes is increased.
The prior method of making junctions between head components does not provide a precise overlap of a flux guide over a front and/or back end portion of the MR sensor. The reason for this is because the prior art employs two resist masking steps. Under the best of conditions the alignment of a critical edge of the resist mask from a benchmark on a wafer is within +/-0.1 .mu.m. Another problem arises from unpredictable shrinkage of the resist. The location of the critical edge of the resist due to shrinkage varies +/-0.1 .mu.m. Even when windage is employed to attenuate the shrinkage problem, shrinkage is still variable from wafer to wafer. Accordingly, when prior art methods place the overlap 0.1 .mu.m over the back edge of the MR sensor, the result can be an overlap of from 0.3 .mu.m on the MR sensor to 0.2 .mu.m off the sensor. Using the square root of the sum of the squares, the standard deviation for the overlap is 0.173 .mu.m. For any wafer containing multiple MR heads with flux guides constructed according to the prior art, the yield will be unacceptably low because of the variability in overlap between the MR sensor and the flux guides. Accordingly, there is a strong felt need for a method of making junctions between MR sensors and flux guides which have a predictable and repeatable overlap. It is anticipated that the MR sensor height in future heads will be as low as 0.5 .mu.m. Alignment in these heads will be even more critical than that required for the present heads with an MR sensor height of 1.0 .mu.m.
Another problem with the prior art process which employs two resist masking steps is the high risk of overmilling the insulation layer during the second resist mask step. This can produce shorting between the MR sensor and the flux guide material layer.
A further problem with flux guides is lack of stabilization in order to prevent Barkhausen noise. The MR sensor, which has a layer of magnetic material, is typically stabilized by a pair of hard bias layers adjacent to its side edges, the side edges extending perpendicular to the ABS. The hard bias layers longitudinally bias the MR sensor parallel to the ABS and stabilize the MR sensor from a multi-magnetic domain state to a single magnetic domain state. Accordingly, upon the termination of flux incursions into the MR sensor, the sensor always returns to a stabilized single magnetic domain state. Without longitudinal biasing the domain walls of multi-magnetic domains shift positions within the sensor, causing Barkhausen noise. This decreases the signal to noise ratio. The same stabilization is necessary for the flux guide because the flux guide material layer is also a magnetic material. In the prior art separate hard bias layers were dedicated for stabilizing the flux guide.